The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Industrial coating of metal articles that will be used in corrosive environments may include application of one or more inorganic and organic treatments and coatings. Steel automotive vehicle bodies and parts, for instance, have an aqueous phosphate coating material applied, are rinsed with rinse water after phosphating, then have an aqueous electrodeposition (or electrocoat) coating applied, followed by multiple aqueous rinses before the electrodeposited coating is cured in an oven.
Electrodeposition coating compositions and methods are widely used in industry today. One of the advantages of electrocoat compositions and processes is that the applied coating composition forms a uniform and contiguous layer over a variety of metallic substrates regardless of shape or configuration. This is especially advantageous when the coating is applied as an anticorrosive coating onto a substrate having an irregular surface, such as a motor vehicle body. The even, continuous coating layer over all portions of the metallic substrate provides maximum anticorrosion effectiveness.
Electrocoat baths usually comprise an aqueous dispersion of a principal film-forming polymer or resin (which terms are used interchangeably), such as an acrylic or epoxy resin, having ionic stabilization. In automotive or industrial applications for which hard electrocoat films are desired, the electrocoat compositions are formulated to be curable compositions. This is usually accomplished by including in the bath a crosslinking agent that can react with functional groups on the principal resin under appropriate conditions (such as with the application of heat) and thus cure the coating. During electrodeposition, coating material containing an ionically-charged resin having a relatively low molecular weight is deposited onto a conductive substrate by submerging the substrate in an electrocoat bath having dispersed therein the charged resin and then applying an electrical potential between the substrate and a pole of opposite charge, for example, a stainless steel electrode. The charged coating material migrates to and deposits on the conductive substrate. The coated substrate is then heated to cure the coating.
As the resins and pigments are plated from the electrocoat coating bath, the bath must be replenished by adding more of the resins and pigments. In one case, resin concentrate and pigment dispersion concentrate are added separately; in another case, a pigmented resin concentrate is added. In both cases, it is beneficial to supply the concentrates with a reasonably low amount of water. The solids content of the concentrate depends on the viscosity profile of the concentrate, but in general the solids content may be raised to 40 to 55 percent by weight nonvolatiles. In general, the solids content is selected to reduce the volume of material to the extent feasible and/or to ensure stability of the dispersed pigment.
On the other hand, manufacturing the resin emulsion is facilitated by including more water that would be desirable in the emulsion concentrate or pigmented emulsion concentrate. For instance, it is desirable to have an excess of water in making the emulsion for azeotropically distilling out certain low-boiling organic solvents. Such low-boiling organic compounds are used as solvents and liquid media in preparation of the components used in electrocoat baths, for example in preparing the film-forming resins and crosslinking agents. Organic solutions of the electrocoat components are dispersed or emulsified in water. (The terms “emulsion” and “dispersion” are being used interchangeably to refer to such waterborne organic components.)
The volatile organic compounds and excess water may then be removed by vacuum distillation at an elevated temperature, for example from at 100-120° F., with agitation or circulation. This process requires that the emulsions be held at the elevated temperatures for lengthy times, particularly because the removal rate of the volatile organic compounds slows as the emulsion becomes more concentrated from removal of both volatile organic compounds and water. The vacuum distillation is continued until the electrocoat resin emulsion reaches a desired solids concentration, adding significant heat history to the product.
The process of concentrating the unpigmented electrocoat emulsion, however, is lengthy and costly. The vacuum distillation may take 25 or 30 hours, tying up equipment and increasing production costs. It would thus be desirable to introduce an improved way of concentrating electrocoat emulsions by removing water during production of electrocoat compositions.